This invention relates to a railroad car-mounted track analyzer suitable for measuring the shape and relative position of the two rails from a car moving along the railroad track. Some analyzers measure the motion of the car as it moves along the rails or the motion of its trucks, and relate such motion to the shape of the tracks. Others measure the distance from points of the car to the rail. Since the body of the car, being supported by springs, moves laterally and vertically by quite a few inches relative to the rails, particularly on curves, the range sensors generally are mounted on the trucks which follow the rail quite closely.
The measurement of the track gauge requires only two range sensors, one for the left and one for the right rail. For measuring the horizontal straightness or curvature of the rail, three range sensors, spaced apart in longitudinal direction, are mounted on a bar fastened to the truck for measuring the three horizontal distances. Three such sensors spaced in a vertical direction are used to determine the curvature in vertical direction. The attainable accuracy using these prior analyzers is moderate. In order to prevent unmanageably large distance variations in curves, quite short longitudinal distances between the range sensors are chosen, say, less than 10 feet.
All tracks have some structural features making the design of the range sensors difficult, such as guide rails used in curved track and at railroad crossings and switches. These guide rails limit the space available for conventional horizontal range finders to less than two inches. Furthermore, fast moving cars tend to lift small rocks from the road bed which can damage the sensors.
Various techniques are used for sensing the range to a rail. Usually a light projection system is utilized where a light beam is directed against a light receiving unit or masking element and either the support for the light source, light receiving unit or masking element is mounted on a frame carrying rollers which must be kept in contact with the rails. However, measurements made with such a range sensing system are frequently inaccurate or difficult to make because the rollers tend to bounce and are subject to wear. A contact-free type of sensor has been developed which measures the electrical capacity between truck-mounted metal plates and the rails. This is a very noise-sensitive instrument not suitable for electrically operated railroads, and the relationship between capacity and range is highly nonlinear unless the range variations are very small.